Recreational trailers are frequently parked on sloping terrain comprised of dirt, gravel, sand or asphalt. Uneven ground results in one wheel of the trailer being lower than the opposite side. A level trailer is much more comfortable and usable with appliances, sink drains, tables, berthing, etc.
Today, as in the past, heavy and inconvenient hydraulic jacks are still used to level. Tall manual hydraulic bottle jacks are difficult to place under the trailer, and with unstable ground, tend to fall over. Even when the trailer is elevated with a jack, the wheels of the trailer need to be blocked with chocks to prevent movement. Because of constant movement of people within the trailer, there is also a tendency to cause jacks to fall.